feat: Expose metrics about used TeamForCapella license usage

A new gauge metric `t4c_licenses` was added to the `/metrics` endpoint,
which contains the number of currently used licenses per TeamForCapella
instance. The instance name is used as label for the metric.

Note: If you have multiple TeamForCapella server instances, which point
to the same license server, this is not detected. Therefore, make sure to
filter the `t4c_licenses` for one instance first.

backend/capellacollab/__main__.py

@@ -13,7 +13,8 @@
 from fastapi.middleware import cors
 
 import capellacollab.projects.toolmodels.backups.runs.interface as pipeline_runs_interface
-import capellacollab.sessions.metrics
+import capellacollab.sessions.metrics as sessions_metrics
+import capellacollab.settings.modelsources.t4c.metrics as t4c_metrics
 
 # This import statement is required and should not be removed! (Alembic will not work otherwise)
 from capellacollab.config import config
@@ -84,7 +85,8 @@ async def shutdown():
     on_startup=[
         startup,
         idletimeout.terminate_idle_sessions_in_background,
-        capellacollab.sessions.metrics.register,
+        sessions_metrics.register,
+        t4c_metrics.register,
         pipeline_runs_interface.schedule_refresh_and_trigger_pipeline_jobs,
     ],
     middleware=[

backend/capellacollab/settings/modelsources/t4c/metrics.py

@@ -0,0 +1,39 @@
+# SPDX-FileCopyrightText: Copyright DB InfraGO AG and contributors
+# SPDX-License-Identifier: Apache-2.0
+
+import typing as t
+
+import prometheus_client
+import prometheus_client.core
+
+from capellacollab.core import database
+
+from . import crud, interface
+
+
+class UsedT4CLicensesCollector:
+    def collect(self) -> t.Iterable[prometheus_client.core.Metric]:
+        metric = prometheus_client.core.GaugeMetricFamily(
+            "t4c_licenses",
+            "Currently used T4C licenses per registered TeamForCapella instance.",
+        )
+
+        with database.SessionLocal() as db:
+            instances = crud.get_t4c_instances(db)
+
+        if not instances:
+            return
+
+        for instance in instances:
+            try:
+                t4c_status = interface.get_t4c_status(instance)
+                used_licenses = t4c_status.total - t4c_status.free
+            except:  # pylint: disable=bare-except
+                used_licenses = -1
+
+            metric.add_metric([instance.name], used_licenses)
+        yield metric
+
+
+def register() -> None:
+    prometheus_client.REGISTRY.register(UsedT4CLicensesCollector())